Integrated circuits are produced from wafers of semiconductor material. The wafers are typically housed in a cassette having a plurality of closely spaced apart slots, each of which can contain a wafer. The cassette is typically moved to a processing station where the wafers are removed from the cassette, placed in a predetermined orientation by a prealigner or otherwise processed, and returned to the cassette, or another cassette, for moving to another location for further processing.
Various types of wafer handling devices are known for transporting the wafers to and from the cassette and among processing stations. Many employ a robotic arm having a spatula-shaped end that is inserted into the cassette to remove or insert a wafer. The end of the robotic arm is referred to as an end effector that typically employs a vacuum to releasably hold the wafer to the end effector. The end effector typically enters the cassette through the narrow gap between a pair of adjacent wafers and engages the backside of a wafer to retrieve it from the cassette. The end effector must be thin, rigid, and positionable with high accuracy to fit between and not touch the closely spaced apart wafers in the cassette. After the wafer has been processed the robotic arm inserts the wafer back into the cassette.
Unfortunately, transferring the wafer among the cassette, robot arm, and processing stations, such as a prealigner, may cause backside damage to the wafer and contamination of the other wafers in the cassette because intentional engagement as well as inadvertent touching of the wafer may dislodge silicon particles that can fall and settle onto the other wafers. Wafer backside damage can include scratches as well as metallic and organic contamination of the wafer material. Robotic arms and prealigners that employ a vacuum to grip the wafer do minimize backside damage and particle creation. Nevertheless, when handling large wafers having small features, even the few particles created are sufficient to contaminate adjacent wafers housed in the cassette. Reducing such contamination is particularly important to maintaining wafer processing yields, which is particularly true for large wafers.
Furthermore, robotic arms and prealigners that grip a wafer with a vacuum have heretofore been limited to handling wafers having diameters smaller than 200 millimeters ("mm"). Semiconductor production systems may soon utilize 300 mm diameter wafers, with larger diameter wafers under consideration. Robot arms and prealigners employing vacuum gripping may be incapable of securely handling these larger wafers quickly and accurately. Because increasing semiconductor yield "is the name of the game, " it is important to grip such larger wafers securely so they can be prealigned quickly and accurately while minimizing wafer backside damage and particulate contamination.
What is needed, therefore, is a specimen gripping end effector that can securely, quickly, and, accurately transfer 150 mm diameter and larger semiconductor wafers to and from a cassette while minimizing backside damage and silicon particle contamination.